Re: PROBLEM: udf mount takes forever to fail + proposed solution

[Jan Kara]

  Hello,

On Thu 10-10-13 23:23:11, Péter András Felvégi wrote:
> recently I made the mistake trying to mount an unformatted ssd
> partition. The mount command 'hang', was unable to kill it. Top showed
> the process is in the uninterruptible D state. However, iotop showed
> slight activity, about 4M/s read from the disk that noone else used.
> This was 100% reproducible. sync froze, too, if was given out after
> the mount cmd. When trying to shut down the machine, it didn't stop,
> just waited for something to happen.
> 
> I narrowed down the problem to the UDF filesystem driver. In
> fs/udf/super.c, udf_check_vsd() reads the sectors in a for loop, with
> the following exit conditions:
> - NSR02 or NSR03 descriptor is found
> - the read fails
> - vsd->stdIdent[0] == 0
> 
> Browsed through the UDF 2.6 spec, ECMA 167 and 119. As I understand,
> the descriptors should start at offset 32768, forming a contiguous
> sequence. In ECMA 167 it is stated that the sequence is terminated by
> an invalid descriptor: unrecorded, or blank (all zeros). However, this
> presupposes that the filesystem is UDF.
> 
> Since the ssd partition was not formatted, it contained only 0xff
> bytes, thus none of the exit conditions were met, and the function
> read through the whole, in two passes. The runtime was pathetic, it
> took the mount 350 minutes to fail. I have no clue why this was so
> slow, reading through the partition with dd gives 482 secs for the
> 220G, ~450M/s. Setting the blocksize to 512 or 2048 didn't make much
> of a difference.
> 
> I peppered the code with some messages to see what happens:
> # time mount -t udf /dev/sdb3 /media/floppy
> UDF-fs: check_vsd: sectorsize=2048
> UDF-fs: check_vsd: sector offs=32768, s_blocksize=512, s_blocksize_bits=9
> UDF-fs: read 107989660 sectors of total size 55290705920 bytes
> UDF-fs: warning (device sdb3): udf_load_vrs: No VRS found
> UDF-fs: Rescanning with blocksize 2048
> UDF-fs: check_vsd: sectorsize=2048
> UDF-fs: check_vsd: sector offs=32768, s_blocksize=2048, s_blocksize_bits=11
> UDF-fs: read 107989660 sectors of total size 221162823680 bytes
> UDF-fs: warning (device sdb3): udf_load_vrs: No VRS found
> UDF-fs: warning (device sdb3): udf_fill_super: No partition found (1)
> mount: wrong fs type, bad option, bad superblock on /dev/sdb3,
>        missing codepage or helper program, or other error
>        In some cases useful info is found in syslog - try
>        dmesg | tail  or so
> real    352m4.740s
> user    0m0.000s
> sys     27m23.560s
> 
> Tried to mount other partitions, too, formatted to ext3, ext4, btrfs
> and ntfs. The mount failed with those sooner, accidentally just
> because there were some blocks near to the beginning with a zero byte
> just at the right place.
> 
> Then I prepared an 'all 0xff' 4G image, and burnt it to a DVD. The
> mount failed, but took only 25 minutes. 'Only', compared to the case
> with the ssd. This truely doesn't reflect the throughput of the
> devices, hopefully someone with more experience will have a clue.
  Thanks for the report and detailed analysis. Frankly, instead of your
function checking the identifier, I'd rather follow the standard in detail
and add handling (meaning ignore) of the remaining specified descriptors
(CDW02, BOOT2) and bail out if anything else is found. If someone complains
because some broken medium stops mounting, we can try something more
elaborate but for now I'd go with the simple solution.

Also please read Documentation/SubmittingPatches - your patch was missing a
changelog entry (you can basically take your somewhat shortened email for
that) and a Signed-off-by line. Thanks!

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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